I have this cheap MXQ Pro 4k box running Android 5.1. It comes preloaded with the manufacturer's launcher, and the Google Play Store shows Leanback as incompatible.
I was, however, able to sideload Leanback 1.1.4 and it works reasonably well. The only thing that doesn't work is Leanback's recommendations.
What is required for the recommendations to appear? Is there a service which provides this information?
It is missing the part of the operating system which you do not have. You need a real Android TV OS.